The Cause:

When I took a detailed look on this issue I saw that the head tag in my template had a Runat="Server" attribute. When I removed this attribute the output was rendered fine. I'm using a few Ajax Controls on forms that throws an error when it can't find a Head tag with the Runat="Server" attribute, so simply removing it isn't the solution.

The solution, Macro's to the rescue:

With a Macro and an XSLT file this issue can easily be solved and after thinking of it it's also a better solution because it's a better separation between HTML and ASP.Net code. To solve it you just create a Macro that excepts two parameters (keywords and description). Then create an XSLT file that outputs the values like this example:

Now you have exact the same functionality and it will work in any situation. Because I'm using the $ sign before the fieldnames that are passed to the Umbraco. Umbraco will do a recursive search for the values of those properties, just like the recursive attribute does on the Umbraco:item tag.